1. Field of the Invention
This invention relates generally to printing apparatus and, more particularly, is directed to an electrostatic printing apparatus of the ion-flow type.
2. Description of the Prior Art
Electrostatic printing apparatus are well-known in the art. For example, one known electrostatic printing apparatus of the ion-flow type includes an ion generator comprised of an insulated, hollow cylinder open along a segment of its length with a corona wire stretched along the axis of the cylinder. A high voltage is applied to the corona wire and ions generated thereby accumulate in the interior of the cylinder. A back electrode over which the recording paper travels is charged with a voltage having an opposite polarity to the voltage applied to the corona wire, and accordingly, an electric field is produced between the ion generator and the back electrode to cause the ions to flow from the former toward the latter and thereby onto the recording paper.
In order to control the placement of the electrostatic charge on the recording paper, a control electrode is interposed between the ion generator and the recording paper and generally includes a plurality of spaced apertures through which the ions are adapted to flow. Upper and lower electrode layers of the control electrode are applied with appropriate potentials to produce a second electric field through each of the apertures. This second electrode field is combined with the aforementioned electric field existing between the ion generator and back electrode to either prevent the flow of ions through selected apertures or to permit the flow of ions through selected apertures. In other words, the flow of ions through each aperture is controlled in a binary or digital manner whereby ions are either permitted or prevented from flowing through each respective aperture. In the case where the apertures are circular, charged dot areas are selectively formed on the recording paper. When the paper is later processed, powder or ink which is charged to have a polarity opposite to the ion charges deposited on the paper are applied to the paper so that the charged dot areas become blackened and the non-charged areas remain the color of the paper.
With the above arrangement, no shading gradations exist between the black and white areas on the paper. In order to obtain different shadings, a plurality of dots, for example, formed in a 4.times.4 matrix for each pictorial or image element, are used to provide such gradation. Thus, the more dots in each matrix that are blackened, the darker the image element will appear. However, since this method uses a plurality of dots on the paper for each pictorial element to indicate gradations in shading, the resolution of the entire printed image is lowered. The signal processing of such image also becomes difficult. Further, when the speed of advancement of the recording medium is changed or if it is desired to change the resolution of the printed image, the density of the printed dots will change, resulting in a variation in the shading of the entire image.
It is to be noted that generally, the corona wire is arranged parallel and in alignment with the apertures of the control electrode. It has recently been proposed, however, to maintain the corona wire and the control electrode in such parallel arrangement while positioning the control electrode, and the apertures, therein, askew with respect to the corona wire. The purpose of such arrangement is to reduce the displacement pitch between the apertures in the direction of the corona wire to increase the resolution of the printed image in such direction. With such arrangement, however, the distance between the different apertures and the corona wire are different, resulting in the ion flow through each aperture also being different. Consequently, the shading of the recorded image varies in accordance with the distance of each aperture from the corona wire, resulting in a deterioration in the quality of the recorded image.